The present invention generally relates to liquid crystal display devices and more particularly to a liquid crystal panel for use in a liquid crystal projector for modulating an optical beam, wherein defects in the projected image, caused by defects on the liquid crystal panel, are reduced. Further, the present invention relates to a liquid crystal projector in which efficiency of cooling is improved.
Liquid crystal panels are used extensively in compact, portable information processing devices such as laptop computers, as a display device. On the other hand, liquid crystal panels are important also in liquid crystal projectors that project an image on a screen with large magnification. In such liquid crystal projectors, a liquid crystal panel is used for spatially modulating an optical beam passing therethrough.
In a typical full color liquid crystal projector, a white optical beam is divided into three color beams each corresponding to one of the three primary colors by a dichroic mirror, and the three color beams thus produced are caused to pass through corresponding one of three liquid crystal panels for spatial modulation. The three color beams having thus experienced a spatial modulation are then synthesized to form a single optical beam which is projected on a screen. Further, there is a full color liquid crystal projector in which a single optical beam is modulated by a single color liquid crystal panel that carries thereon color filters.
FIG. 1 shows a typical full color liquid crystal projector that uses three liquid crystal panels.
Referring to FIG. 1, a white optical beam produced by a light source 1 impinges upon a dichroic mirror 2, wherein the dichroic mirror 2 separates one of the three primary colors and reflects the same to a first liquid crystal panel 4 via a mirror 3 in the form of a first color beam. On the other hand, the optical beam that has passed through the dichroic mirror 2 impinges upon a next dichroic mirror 8, wherein the dichroic mirror 8 separates the next of the three primary colors and reflects the same to a second liquid crystal panel 9 in the form of a second color beam. Further, the optical beam passed through the dichroic beam 8 impinges upon a third liquid crystal panel 10 in the form of a third color beam.
Thereby, the first color beam experiences a spatial modulation upon passage through the first liquid crystal panel 4, while the second color beam experiences a spatial modulation upon passage through the second liquid crystal panel 9. The first and second color beams having thus experienced respective spatial modulations are merged with each other at a third dichroic mirror 5 that passes the first color beam from the liquid crystal panel 4 to a fourth dichroic mirror 6, while the dichroic mirror 5 reflects the second color beam to the foregoing dichroic mirror 6.
Further, the third color beam experiences a spatial modulation upon passage through a third liquid crystal panel 10, wherein the third color beam having thus experienced the spatial modulation impinges upon the dichroic mirror 6 after reflection by a mirror 11. Thereby, the first through third color beams are synthesized at the dichroic mirror in the form of a single color beam, and the single color beam thus produced is projected upon a screen by a projection lens 7.
Each of the liquid crystal panels 4, 9 and 10 has a construction in which a liquid crystal layer of a twist-nematic (TN) type, a super twist-nematic (STN) type or a polymer dispersion type is sandwiched between a pair of transparent glass substrates. As usual in liquid crystal panels, each of the liquid crystal panels 4, 9 and 10 includes pixel electrodes provided on one of the glass substrates for switching on and off the transmission of the optical beam through the liquid crystal layer. Further, the dichroic mirrors 2, 3 and 8 are formed to have a multilayered structure adapted such that each of the dichroic mirrors separates one of the three principal colors.
In the fabrication process of liquid crystal panels including those for use in such a liquid crystal projector, the glass substrate forming the liquid crystal panel is subjected to various processes such as cleaning, deposition, patterning, and the like, while such processes generally include the step of holding the glass substrate by a vacuum chuck mechanism or a similar holding process that includes a mechanical contact of a hard element or member to the glass substrate.
Thus, as indicated in FIGS. 2A and 2B, there is a substantial risk that the glass substrate is scarred or damaged as a result of such a mechanical contact.
Referring to FIG. 2A showing the liquid crystal panel in a cross sectional view, a liquid crystal layer 11 is sandwiched by a glass substrate 12 and a glass substrate 13, wherein it will be noted that the glass substrate 12 includes a defect 14 on the outer surface thereof as a result of mechanical contact at the time of chucking, and the like. Similarly, the glass substrate 13 carries a defect 15 on the outer surface.
Thus, when the liquid crystal panel is used for modulating the optical beam impinging thereto as indicated in FIG. 2B, the defects 14 and 15 cause a scattering or disturbance of polarization in the optical beam as it passes through the liquid crystal panel vertically to the plane of drawing in FIG. 2B, while such a scattering or disturbance of polarization of the optical beam causes various unwanted effects on the image projected on the screen. For example, when a scattering occurs in the optical beam as a result of such defects, the projected image may include dark spots in correspondence to such defects. When the defects 14 and 15 cause a disturbance in the polarization, on the other hand, there appear dark or bright spots in the projected image due to insufficient transmittance or interruption of the optical beam. Further, when the liquid crystal projector is a color projector, the defects 14 or 15 generally cause a formation of colored spots in the projected image in which the color is changed unwantedly.
Meanwhile, conventional liquid crystal projectors, which use a powerful light source for increased visibility of the projected images, have suffered from the problem of degradation of polarizer or analyzer in the liquid crystal panel because of the heat applied thereto by the high power optical beam. When the polarizer or analyzer is deteriorated as such, the contrast of the projected image is inevitably deteriorated and hence the quality of the projected image.
In order to suppress the temperature rise of the polarizer and analyzer as much as possible, conventional liquid crystal projectors generally use a compulsory cooling system including a cooling fan for cooling the polarizer and analyzer. In liquid crystal projectors, on the other hand, it is essential that the power consumption is small and the size of the projector is compact. Further, the noise level of the cooling fan is required to be as low as possible.
FIG. 3A shows a conventional liquid crystal projector 20 disclosed in the Japanese Laid-open Patent Publication 5-249411.
Referring to FIG. 3A, the liquid crystal projector 20 includes a housing 21 on which a projection lens 22 is mounted. Further, the housing 21 includes therein a light source 23 for producing a white optical beam and a dichroic mirror system 23A similar to the one explained with reference to FIG. 1, wherein the dichroic mirror system 23A separates the optical beams of the three primary colors from the white optical beam produced by the light source 23. Thus, an optical beam of the first primary color is supplied to a first spatial optical modulator 24A including a liquid crystal panel similar to the liquid crystal panel 4 of FIG. 1 for spatial modulation. Similarly, an optical beam of the second primary color is supplied to a second spatial optical modulator 24B including a liquid crystal panel similar to the liquid crystal panel 9 of FIG. 1 for spatial modulation. Further, an optical beam of the third primary color is supplied to a third spatial optical modulator 24C including a liquid crystal panel similar to the liquid crystal panel 10 of FIG. 1 for spatial modulation. The optical beams thus modulated by the spatial optical modulators 24A-24C are synthesized into a single beam by a dichroic mirror system similar to the one explained in FIG. 1, and the optical beam thus synthesized is projected to a screen via the projection lens 22.
In the liquid crystal projector 20 of FIG. 3A, it should be noted that three, separate compulsory cooling systems 25A-25C are provided in cooperation with the respective spatial optical modulators 24A-24C.
FIG. 3B shows the construction of the spatial optical modulator 24A and the cooling system 25A cooperating therewith in detail. It should be noted that the spatial optical modulators 24B and 24C as well as the cooling systems 25B and 25C have essentially the same construction.
Referring to FIG. 3B, the cooling system 25A includes a turbo fan 25a driven by a motor M and a duct 25a' for guiding the air flow caused by the turbo fan 25a. The spatial optical modulator 24A, on the other hand, includes a condenser lens 31 defined by a flat surface at the exit side of an optical beam directed to the spatial optical modulator 24A and a polarizer 32 provided on the foregoing flat surface, in addition to a liquid crystal panel 33 and an analyzer 34, wherein the analyzer 34 is provided on the exit side of the liquid crystal panel 33. Thereby, the condenser lens 31, the polarizer 32, the liquid crystal panel 33 and the analyzer 34 are disposed along a path 35 of an optical beam 30 in a traveling direction of the optical beam 30 with a parallel relationship, such that each of the condenser lens 31, the polarizer 32, the liquid crystal panel 33 and the analyzer 34 intersects the path 35 perpendicularly.
In the construction of FIG. 3B, it should be noted that, while the condenser lens 31 and the polarizer 36 are provided with an intimate contact and the liquid crystal panel 33 and the analyzer 34 are provided with an intimate contact, the polarizer 32 and the liquid crystal panel 33 are spaced apart by a gap 36 having a size W.sub.1 of typically several millimeters in the direction of the path 35, such that the cooling air flow caused by the fan 25a enters into the gap 36 from an inlet part 39. Thereby, the guide 25a' is formed such that the cooling air flow enters the gap 36 in the form of a cooling air flow 38 that forms a small angle .THETA. with respect to a major surface 33a of the liquid crystal panel 33, Thereby, the cooling air flow 38 removes the heat from the foregoing major surface 33a of the liquid crystal panel 33. As a result of such a cooling of the liquid crystal panel 33, the analyzer 34 is also cooled. Further, the cooling air flow 38 forms an air flow 40 directed to the polarizer 32 after reflection at the foregoing major surface 33a. Thereby, the polarizer 32 is cooled by such a reflected air flow 40.
In such a spatial optical modulator, it should be noted that the angle .THETA. between the air flow 38 and the major surface 33a cannot be increased as desired, because of the limited size of an inlet part 39 through which the cooling air 38 is introduced into the foregoing gap 36. As a result, there is a possibility that most of the cooling air may escape straight from the other side of the gap 36, without cooling the liquid crystal panel 33 or the polarizer 32. When this occurs, a part of the polarizer 32 as well as the analyzer 34 may experience an unwanted temperature rise, which eventually reduces the lifetime of the polarizer 32 or the analyzer 34.
Further, when the cooling is not uniform in the polarizer 32 or analyzer 34, the projected image may experience unwanted inhomogeneity.